The survey will not have the power to legalise same-sex marriage but it could lead to a vote in parliament if the majority of Australians support the change.

The postal plebiscite has been criticised by opponents as an unnecessary delay in resolving the debate, which they say could be settled immediately in parliament.

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Mr Turnbull said he did not believe same-sex marriage would threaten "traditional" marriage

The Prime Minister previously said he would personally be voting in favour of same-sex marriage, but has not publicly campaigned prior to the surprise speech on Sunday.

"Throughout my public life I've sought to ensure same-sex couples are not discriminated against and their entitlements, be it in respect of medical benefits, taxation, superannuation or employment, are no different to those afforded to heterosexual couples. Why then shouldn't those same rights now be extended to marriage?"

Mr Turnbull pointed out that 23 other countries had already legalised same-sex marriage.

"In any one of those nations, has the sky fallen in? Has life as we know it come to a halt? Has traditional marriage as we know it been undermined? The answer is no," he said.

Meanwhile, Mr Shorten, who is the leader of the Australian Labor Party, told campaigners he anticipated the yes vote to win in the postal survey.

"We've got one last mountain to climb before we make marriage equality a reality. Let's climb it together, today," he said.

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Mr Shorten and Labor support marriage equality, but have criticised the plebiscite vote

His party have been opposed to the plebiscite vote launched by Mr Turnbull's government, criticising its A$122m (£75m; $97m) cost, its non-binding nature, and arguing it could prompt hate-filled campaigns.